If anybody (e.g. Valve or Apple) would see the Mac as a viable gaming platform, they would offer an API wrapper solution like Proton (which would emulate DirectX and Win32 APIs on top of macOS APIs). But it's already starting at the hardware, embedded Intel GPUs don't cut it for gaming.
But the 3D API is just one tiny piece when porting games to other platforms, and the importance of cross-platform 3D-APIs is overrated. Game consoles always had their own non-standard 3D-APIs, and this didn't hinder games being created or ported. And OpenGL only was a cross-platform API in theory, in practice every OpenGL implementation had its own peformance and compatibility quirks, and Vulkan apparently isn't much different.
But the root cause is not a technological problem, there simply are not enough people interested in gaming on macOS. If there would be a market, games would come to macOS, with or without Apple's involvement.